This invention relates to an electrical safety device of the kind commonly known as a ground fault circuit interrupter residual current device or earth leakage circuit breaker (ELCB), which latter term will be used herein.
An earth leakage circuit breaker generally makes use of a relay with two-pole normally-open contacts. A holding circuit maintains the relay closed so long as voltage appears at the output of the device, and a sensor arranged to sense an unbalance in current disables the holding circuit and causes the relay to open on the occurrence of any leakage current to earth. Resetting means must be provided to close the relay after it has opened, or when the circuit is first switched on, and some means for testing the device is necessary to ensure that it remains operative and sufficiently sensitive to trip under fault conditions. These functions are normally provided by means of a reset button and a test button on the casing of the device. It will be noted that in the conventional earth leakage circuit breaker the contacts will usually make or break the full load current when they close on resetting or open on testing. cl SUMMARY OF THE INVENTION
The present invention provides an earth leakage circuit breaker in which the resetting and test functions are performed automatically on plugging in or unplugging the appliance to be protected. This obviates the need for test and reset buttons, and also ensures that the testing and resetting are carried out when no load current is being drawn, thereby lengthening the life of the relay contacts, or allowing contacts of lower rating to be used.
In a more elaborate version of the breaker of the invention, insertion of the plug may first re-set the breaker, then initiate a test trip, and then reset the breaker only if the initial test trip has been successful.